It is well known that mechanical loading increases formation and remodeling of bone. Bone mass is increased in response to exercise, while chronic unloading of bone, such as occurs during prolonged bed rest and in microgravity during space flight, results in atrophy of bone. The broad aim of our research is to understand the cellular and molecular mechanisms that regulate mechanically-induced bone formation. Experimental studies suggest that mechanically-induced bone formation may be stimulated by fluid shear stress (FSS)-induced activation of osteoblasts. Mechanical activation of osteoblasts is thought to result from the movement of interstitial fluid through the porous spaces inside bone that subjects osteoblasts to FSS during high impact loading. However, the cellular mechanisms through which FSS promotes an anabolic response in osteoblasts are not clearly understood. Interestingly, a large proportion of osteoblasts at sites of bone remodeling are destined to undergo programmed cell death (apoptosis). Therefore, processes that inhibit osteoblast apoptosis may be effective in increasing bone formation and improving bone strength in normal and disease states. Our preliminary studies indicate that mechanical stimulation of osteoblasts in vitro, by exposure to steady fluid shear stress, inhibits osteoblast apoptosis. Therefore, in this application we propose experiments that are designed to investigate the signaling mechanisms through which FSS promotes the survival of osteoblasts. We will: (1) determine the mechanisms through which FSS regulates intracellular signaling pathways involved in control of apoptosis, and (2) determine the role of temporal shear gradients in the anti-apoptotic response of osteoblasts to FSS. The long-term goal of this research is to identify strategies for improving bone health by better understanding the cellular and molecular mechanisms that regulate osteoblast survival. In this application, we propose to use an in vitro cell culture model to test the hypothesis that exposure of cells to either steady or pulsatile fluid shear stress regulates osteoblast survival through specific cellular processes, including activation of the PI3-kinase/Akt and MAPK signaling pathways and inhibition of caspase activation. We will use primary cultures of rat calvarial osteoblasts, and osteoblast cell lines, including MC3T3-E1 and UMR106.01 cells, to investigate the cellular mechanisms that regulate apoptosis. [unreadable] [unreadable]